1. Field of the Invention
The present invention is directed to polyimide oligomers and polymers and, more particularly, to polyimide oligomers incorporating hexafluoroisopropylidene groups in the backbone thereof and being terminated at each end by a selected unsaturated group, to the corresponding polyamic acid oligomers, and to polymers of such oligomers.
2. Description of Related Art
Polymers formed from polyimide oligomers are well known in the art. U.S. Pat. No. 3,845,018, assigned to the present assignee, discloses a polyimide oligomer containing acetylene end groups, through which the oligomer cures by an addition reaction without the evolution of gas and the formation of unwanted voids in the cured polymer. This material is commercially available under the trade name THERMID.RTM., from National Starch and Chemical Corporation. The polymers of this oligomer are described in U.S. Pat. No. 3,879,349, assigned to the present assignee. These oligomers and polymers contain poly(arylene ether) or poly(arylene thioether) groups in the backbone thereof. In the past, these polymers have been useful for forming high temperature resistant films and laminates. More recently, polyimide materials have been found useful in the fabrication of semiconductor devices as a dielectric layer to provide electrical insulation between adjacent conductive materials in multilayer structures. One such method for using these polyimide polymers is disclosed in U.S. Pat. No. 4,654,223 to Araps et al.
In order to improve the solubility of polyimides known in the early development of these materials, it was found desirable to incorporate hexafluoroisopropylidene groups in the polymer chain, as disclosed in U.S. Pat. No. 3,356,648 to Francis E. Rogers. In the method of Rogers, a tetracarboxylic acid anhydride containing a hexafluoroisopropylidene group is reacted with a hexafluoroisopropylidene bridged diamine in a solvent to form a polyamide-acid, which is then converted to the polyimide by heating and/or treatment with a dehydrating agent to produce cyclization. It is known in the art that incorporation of hexafluoroisopropylidene (6F) units into a polymer backbone structure assists in the processing of high temperature resistant aromatic heterocyclic polymer systems. These 6F units tend to lower the glass transition temperature (Tg) of polymers prepared therewith and thus improve their melt characteristics. In addition, 6F units within a polymer chain prohibit extensive conjugation of aromatic moieties, thereby providing good electric insulation characteristics for the end product resins. Such lack of conjugation also reduces UV-visible absorption, thus allowing end product polyimides to appear colorless and to resist photochemical degradation.
As noted above, the polymers of Rogers were developed to provide improved solubility and were contemplated for use as high-temperature resistant films and coatings, rather than in the fabrication of semiconductor devices. I have subsequently found that the polyimide of Rogers has a low dielectric constant (about 2.5) which is desirable for use as an interlevel insulator in a semiconductor device in order to reduce signal loss due to the capacitance effect. However, these polyimides were also found to have appreciable solubility in solvents such as acetone, glycol ethers, and dimethylacetamide. Consequently, for multiple layer applications, such as in integrated circuits and hybrid circuits, which require subsequent chemical processing after the polyimide layer has been formed, this polyimide may be difficult or impossible to use and can result in delamination of the final structure.
Thus, a need exists in the art for a polyimide polymer suitable for use as a dielectric layer in a multilayer semiconductor device, which has a low dielectric constant, and practical solvent resistance.